A common feature of tumor cells is the energy required to maintain cell proliferation through metabolic reprogramming[18]. Lipids not only constitute the basic structure of biological membranes but also function as signaling molecules and energy sources. Fatty acids are an important component of lipids, and studies have shown that tumor cells can support their rapid proliferation, migration, invasion, and metastasis through increased fatty acid uptake[19, 20]. CC is one of the malignant tumors with high morbidity and mortality. Exploring the role of different fatty acid metabolism genes in CC plays an important role in improving the therapeutic efficacy of CC and specifying individualized treatment.
In this study, we summarized fatty acid metabolism genes through three gene sets. Univariate and multivariate Cox proportional risk regression analyses were performed to screen out six fatty acid metabolism genes (ENO3、ELOVL3、ACOT11、ALAD、ELOVL6、ACADL) closely related to prognosis. The key genes related to fatty acid metabolism were further screened by LASSO regression and cross-validation, and the prognostic model was constructed. These six fatty acid metabolism genes were abnormally expressed in multiple tumor tissues and were closely related to tumor progression. However, the role of these genes in the prognosis of CC remains to be studied. ENO3 is an enzyme that catalyzes the formation of phosphoenolpyruvate from 2-phosphoglycerate. ENO3 can promote the proliferation and migration of colorectal cancer by enhancing cell glycolysis[21]. ELOVL3 and ELOVL6 are rate-limiting enzymes that catalyze the synthesis of very long-chain fatty acids. Dysfunctional ELOVL3 has been reported to be associated with various, including cancers. In prostate cancer, transcription enhancement of ELOVL3 is closely related to the invasion and migration of prostate cancer[22]. ELOVL6 promotes cell proliferation and Akt activation, enhancing carcinogenic activity in hepatocellular carcinoma (HCC), and is associated with poor prognosis in HCC patients[23]. ACOT11 can encode enzymatic hydrolysis of fatty acid acyl CoA esters into free fatty acids and CoA. The high expression of ACOT11 in lung cancer patients is positively correlated with poor prognosis. ACOT11 affects tumor invasion and migration and promotes cell apoptosis and cell cycle arrest through various signaling pathways[24]. Overexpression of ALAD in breast cancer patients can inhibit epithelial-mesenchymal transition phenotype, and inhibit breast cancer proliferation through TGFβ -mediated signaling pathway[25]. ACADL, as a key enzyme in the first step of mitochondrial fatty acid β oxidation, is down-regulated in liver cancer, and has a tumor-suppressor effect through Hippo/YAP signaling pathway[26]. ACADL can inhibit the expression of MMP14 through the STAT3 signaling pathway, thus inhibiting the metastasis of HCC[27]. However, the relevant role of this gene in fatty acid oxidation in CC is unclear. In this study, ACADL expression was down-regulated and correlated with the prognosis of CC patients. It was incorporated into the construction of the prognostic model for HCC patients, further demonstrating the important role of this gene in the development and progression of colon cancer.
The prognostic model was constructed based on the six genes mentioned above, and the patients were divided into two groups of high and low risk according to the median risk score. The combined PCA, ROC, and risk curve analysis suggested that the prognostic model had a good predictive value. Subsequently, risk score and clinical indicators were integrated (age, stage, T status, N status, M status), and the results suggested that risk score was an independent prognostic factor, which further confirmed that the model could predict the prognosis of CC patients.
Next, we further analyzed the function of immune cells in the high and low-risk populations, and the results showed that the immune cells in the high-risk group were more activated, and the related functions of immune cells were more active. Specifically, the expression level of macrophages and T helper cells was higher, and the expression level of Tregs was lower in the high-risk population. This may be related to the overactivation of the immune system in patients with early colon cancer. Previous research has shown that the increase of M2 polarization in macrophages can promote the growth of colon cancer[28–30]. Salman found that in the CC microenvironment, the infiltration of highly immunosuppressed Treg cells was significantly increased, and these increased Tregs may hinder the response of CRC patients to immune checkpoints blockade. However, the effects of different immune checkpoints inhibitory mechanisms on Treg level or activity need to be further studied[31]. Thus, targeting these immune cells offers the possibility of immunotherapy for colon cancer.
PD-1 (PDCD-1), PD-L1 (CD274), LAG3, and CTLA4 are important immune checkpoints. Abnormal activation of immune checkpoints on tumor cells contributes to the immune escape of tumor cells. Immune checkpoint inhibitors have been applied increasingly applied in various malignant tumors, proving their therapeutic potential. In melanoma patients[32], ipilimumab (CTLA4 inhibitor) and nivolumab (PD-1 inhibitor) have a significant survival benefit, and nivolumab plus ipilimumab has a more durable survival benefit. Our results showed that the expression levels of PD-1, LAG3, and CTLA4 were higher in high-risk patients, suggesting that targeted immune checkpoints such as PD-1, LAG3, and CTLA4 may improve the efficacy of immunotherapy in CC patients. Finally, TIDE scores of the high and low-risk groups were calculated respectively[33], and the results showed that the probability of immune escape was higher in the high-risk group, and the therapeutic effect of immune checkpoint blockade was worse.
Finally, we incorporated the prognostic score into the prediction model, and a nomogram was constructed. The Nomogram prediction model's accuracy was evaluated using calibration plots, ROC curve, and DCA. DCA is a new method to evaluate clinical predictive models[34]. The above three methods demonstrate the practicality of the prognostic model, which can help us timely identify patients with poor prognoses and specify individualized treatment plans to further improve the survival rate of CC patients.
In summary, we constructed prognostic scores based on six fatty acid metabolism-related genes, and a nomogram was constructed by incorporating prognostic scores into the prognostic model. In addition, we further explored differences in immune cell infiltration, immune checkpoint activation, and immunotherapy effects among patients at high and low risk. Our results suggest that fatty acid metabolism-related genes can be used as a new therapeutic target for CC and further improve the survival rate of CC patients through individualized therapy.